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How do leaves grow flat? Marcus Heisler (EMBL Heidelberg)

News / 17 October 2016

The Heisler Group focuses on investigating fundamental plant development questions. Specifically, they are seeking answers to some fundamental biological questions such as:

• How do plants position their organs such as leaves or flowers?

• How do they create organs of a particular shape, for instance the flat shape of a leaf?

Marcus explains that a fundamental question in animal and plant biology research is how tissues are organised and patterned. We know that plants evolved independently of animals and so an interesting question is whether plants and animals use the same or different developmental strategies. For example, both plants and animals polarize their cells but whether the mechanism underlying this polarity is similar or not is unknown.

We could also ask the question “how do plants know where to grow a leaf”? Animals use spatial cues laid down during embryo development to position their limbs. Plants on the other hand usually produce leaves continuously in regular patterns, suggesting a different kind of mechanism may be involved. What we also know is that in animals, development is often controlled by specialised tissues called organisers, and often these organisers get positioned at boundaries between different cell types. In fact, experimentally, all you have to do is place these different cell types next to each other and an organiser forms, leading to the creation of a whole new organ such as a limb bud or wing. Surprisingly plant leaves are thought to develop in a similar way.

When a leaf develops, we know that two main tissues form, one on the top side, facing the sun, and the other on the bottom side, with an organizer thought to be located in between. Both the top and bottom tissues are therefore needed for an organizer to form to ensure that the leaf grows flat.

Earlier this month Marcus and one of his post-docs in the Heisler EMBL Heidelberg laboratory, Paz Merelo published a paper in PNAS showing how different types of molecules on the top and bottom sides of a leaf keep each other in check, ensuring the leaf grows flat. This work was done in collaboration with EMBL, University of Sydney and Stephan Wenkel’s laboratory at the University of Copenhagen.

Cells in the upper side of the leaf produce proteins known as Class III HD-ZIPs, which are suppressed by microRNA 165/166 molecules, which are produced in the bottom tissues. The question asked by Marcus and Paz was, ‘how are these microRNA’s restricted to the bottom side’?

Using the model plant Arabidopsis, Paz Merelo examined plants in which a certain set of proteins, the Class II HD-ZIP proteins (related to the Class III HD-ZIPs), were not functioning and found that the microRNAs where no longer restricted to bottom tissues. Using confocal microscopy, Paz found a similar situation when the Class III HD-ZIPs weren’t functioning. In fact, Paz found that both Class II and Class III HD-ZIPs are needed together and physically interact within the top cells to prevent these microRNAs from turning on. So in the absence of the Class IIs, the Class IIIs alone cannot repress the microRNAs and the same if the Class IIIs are missing, leading to leaves that grow stem-like instead of being flattened. A two-way antagonism between microRNA 165/166 molecules and Class II/III HD-ZIPs therefore keeps top and bottom cell types balanced.

How is the balance kept just right? How does the organizer actually ensure that the leaf grows flat? Does the organizer control cell polarity? These are some of questions the Heisler lab has also tackled, so stay tuned…